Dropdowns to child pages from parent pages are not currently a feature of the Spectrum theme. If you would like to make that suggestion directly to Staff here's the link. http://en.support.wordpress.com/contact/

To clarify: By now, all themes that feature a top nav menu support dropowns if you replace their standard menu with a custom one. But only some of those themes support dropowns in their standard menu, and Spectrum isn't one of them.

I see the following message under "theme locations" in the "Menus" screen:
"The current theme does not natively support menus, but you can use the “Custom Menu” widget to add any menus you create here to the theme’s sidebar."